Human chorionic gonadotropin (hCG) is a heterogeneous glycoprotein hormone comprising an alpha-subunit and beta-subunit that can vary in peptide and carbohydrate structure. After conception, hCG produced by early trophoblast cells acts on luteinizing hormone (LH)/hCG receptor corpus luteum cells to promote progesterone production and establish maternal recognition of pregnancy. hCG is not simply 1 molecule, and 2 variants of hCG appear to have independent activities in promoting tumor cell growth, invasion and malignancy. Hyperglycosylated hCG (H-hCG), produced by cytotrophoblast cells, is a marker for cytotrophoblast cells and tumor marker for gestational trophoblastic diseases. H-hCG promotes growth and invasion in these cells during pregnancy implantation, and growth in varying degrees by many nontrophoblastic neoplasms. beta-hCG is a marker of poor prognosis shown to promote growth and invasion in vitro, suggesting autocrine growth factor properties. Vaccines to b-hCG have been successfully demonstrated, suggesting a potential adjuvant therapy in cancer treatment. Although sufficiently distinct in both structure and occurrence, similarities have been observed between H-hCG and beta-hCG as promoters of cell growth, invasion and malignancy. It is somewhat irregular for 2 structural variants of a molecule to have independent actions, actions very different to the gonadotropic function of the established hormone hCG. (J Reprod Med 2008; 53:499-512)